1. Field of the Invention
The present invention relates to a method of chemical sand control in wells drilled through subterranean formations. More specifically, the method comprises instantaneous application of pressure to drive a resin from a wellbore into the formation, the pressure being greater than a selected value, and subsequently driving the resin into the formation. Perforating means or explosives in the wellbore may subsequently be used to assist in driving the resin into the formation.
2. Description of Related Art
Hydrocarbons are often found in unconsolidated rocks. These rocks are usually relatively young in geologic age, and are unconsolidated because natural processes have not cemented the rock grains together by mineral deposition. Vast amounts of oil and gas are produced from these unconsolidated rocks, the most notable production being along the Gulf Coast of the United States, in the Gulf of Mexico, in the North Sea and in several other areas of the world.
To produce oil and gas from these reservoirs, it is necessary to employ "sand control" methods in the wells. There are two types of sand control methods--mechanical and chemical. Mechanical methods employ filter-type devices which are placed in the well opposite the unconsolidated sand and which prevent movement of rock grains into the wellbore. Such methods are sometimes successful, but the filter materials often plug or erode. Chemical methods employ a liquid resin which is injected from a wellbore into the unconsolidated rock surrounding the well. The resin is catalytically polymerized to form a porous, permeable rock mass. With most resins, it is necessary to inject a displacement fluid, which is a fluid following the resin which is not miscible with the resin. The purpose of this displacement fluid is to reduce the saturation of the resin in the rock to a lower value, which increases permeability to flow through the consolidated mass. The catalyst for the polymerization may be included in the resin or it may be injected in the displacement fluid. Several types of resins are presently used in the sand control art. These include furan, epoxy, phenolic, phenolic-furfuryl, and phenol-formaldehyde resins.
Chemical methods have several important advantages over mechanical methods, but the high cost of the resins and the difficulties in obtaining sufficiently uniform injection of chemicals have limited application to relatively short intervals of perforations. In some unconsolidated reservoirs where production is marginal, the high cost of chemical sand control has prevented recovery of the oil or gas. The paper "Sand Control, Part 7, Consolidating Formation Sand by Chemical Methods Requires Precise Application and Careful Fluid Handling," World Oil, May, 1975, pp. 47-53, is a summary of the many factors that must be considered in chemical sand control.
In conventional oil field practices, soon after a well is drilled, casing is placed in the well and cemented in place through the zone of interest. The casing must be perforated to allow production of fluids from the well. Sometimes perforating means, usually a perforating gun, which may be shaped charge or bullet gun, is used inside casing with no other pipe in the well. More often, tubing is run into the well to the desired depth, the drilling fluid is displaced by another fluid, a packer on the tubing is activated to seal the annulus outside the tubing and above the packer to pressure, and a perforating gun is lowered into the well to the desired depth to perforate the casing. The fluid in the wellbore at the depth to be perforated when the perforating gun is fired is called the "perforating fluid" or "completion fluid." When the casing is perforated without tubing in the well, the pressure in the casing is usually raised to a value greater than fluid pressure in the formation before perforating. This is called "overbalanced perforating." Perforating fluids that do not contain solid particles which plug perforations are normally recommended as perforating fluids.
When tubing is in the well, pressures in the well are often adjusted to make pressure inside the wellbore less than the formation fluid pressure when the well is perforated. This is called "underbalanced perforating." A "through-tubing" perforating gun is used. The technique is used to minimize plugging of the perforations from solids in the wellbore fluid and to assist in clearing perforations of debris. (See Production Operations, Vol. 1, Chap. 7, by Allen, T. O. and Roberts, A. P., Oil and Gas Consultants International, Inc., Tulsa, 1978.) If underbalanced perforating is not used, special back flow and surging techniques have often been recommended to clean the perforations of solids before chemicals are injected for sand control. A sand control resin has not heretofore been considered as a perforating fluid. The effect of producing a well before sand control is practiced is often that sand will be produced and the formation around the well can then collapse or move toward the wellbore. This disturbance of the near wellbore region may have long term detrimental effects resulting from increased flow of solids toward the well.
Prior art methods of injecting sand control chemicals into formations have required pumping large amounts of fluids. Common practice has been to treat a well for sand control soon after perforating, sometimes after a short production time to help clean the perforations. Large volumes of chemical resins are generally necessary to insure that resin enters every perforation. If every perforation is not treated, sand particles may flow from even a single perforation in sufficient quantity to cause the well to cease production. Experts often suggest that resin be injected into the formation in amounts of about 100 gallons per foot of perforations in an effort to treat the formation. The resins cost as much as $40 to $50 per gallon, so the cost of chemicals for the prior art sand control methods is quite high. High costs have prevented economic production of some unconsolidated zones. Also, it is sometimes necessary to control sand production even before a new well can be tested. The high cost of prior art methods of sand control, either mechanical or chemical, can discourage testing of some zones.
U.S. Pat. No. 2,693,856 discloses use of clean fluids (those which will not plug perforations) to replace drilling fluid in wellbores before the casing in the well is perforated. Controlled density fluids such as gelled oils, acid and other gelled liquids are described. The well pressure is controlled by having fluid densities in the wellbore to produce hydrostatic pressures greater than the pressure in the formation to be perforated. After perforating, the clean fluids enter the formation around the well.
U.S. Pat. No. 2,718,264 discloses a method of firing a perforating gun when the gun is surrounded by cement in the cased wellbore. The gun is placed opposite a zone that is to be sealed or isolated from a separate producing interval in the well. The cement is said to be forced practically simultaneously through the newly formed perforations and into fractures in the formation by the force of the perforating process.
U.S. Pat. No. 2,689,008 discloses a method of forcing cement or cementitious material through preexisting perforations by igniting a high explosive in the wellbore above the perforations. A shaped charge directed downward and placed above the perforations is preferred. The purpose of the cementitious material is to permanently plug the preexisting perforations.
U.S. Pat. No. 4,009,757 discloses a method of completing an unconsolidated production zone by perforating into the zone with a polymeric material in the wellbore which sets to form a porous mass in the perforations and the wellbore.
There is a significant need for a method of sand control which provides perforations free of plugging material, places resin for sand control effectively through all perforations and into the formation, and allows the use of smaller amounts of resin and less equipment at a well to decrease costs.